The development of the immune system is under investigation in our laboratory using the South African clawed frog, Xenopus laevis, as a model. For these studies the advantages of this model are the external development of embryos and the lack of interference of the maternal environment during embryonic development. Our past work has involved a characterization of the first immunoglobulin synthesized during developemnt and an investigation of several other properties of the early immune system. In addition, we have prepared a nucleic acid probe for Xenopus heavy chain sequences and used this to measure the onset of immunoglobulin mRNA synthesis during development. We propose, during the period of this application, to characterize the structure and sequence of heavy chain genes from both cells which do not synthesize immunoglobulin and those that do. In this way, we will analyze the contribution of gene rearrangement to the generation of antibody diversity in these amphibians. We will also examine the structure of immunoglobulin mRDA during development with particular reference to the time of appearance of the mRNA for secreted and membrane-bound immunoglobulin forms. In summary, we will continue to study the molecular changes involved in the development of the immune system at a stage where these events are difficult or impossible to study in mammals as they occur in utero.